Natural biological materials provide numerous functional nanostructures and attractive materials. Proteins and peptides represent a fundamental fraction of natural biological molecules that carry out a host of essential functions in biology, including molecular recognition, catalysis, information storage, and controlled crystallization of inorganic materials.
Amino acids are the molecular building blocks of peptides and proteins. An amino acid contains both an amine group and a carboxyl group separated by a single carbon atom termed the α carbon. Attached to the α carbon is an organic substituent, the side chain. Of the natural 20 amino acids, 19 have a chiral α carbon, with the most common chirality being L; however, D amino acids are also found. Alternatively, chiral centers are defined using the R,S system. For α-amino acids, there are two possible R,S configurations and three potential sites for substitution, seen in FIG. 1. Unlike α-amino acids, β-amino acids contain two carbon atoms between the amine group and the carboxyl group. If a side chain bonds to the carbon closest to the amino group, the β-amino acid is termed a β3-amino acid. Similarly, if the side chain bonds to the carbon closest to the carboxyl group, then the amino acid is termed a β2-amino acid. For β-amino acids, there are eight possible R,S configurations and five possible sites for substitution. Various amino acids are illustrated in FIG. 1. It is readily apparent that the possible number of isomers increases significantly from the α-amino acid to the β-amino acid.
In 1993, researchers published the design, synthesis, and characterization of cyclic D,L peptides that self assemble into hollow tube-like structures as confirmed by electron microscopy and electron diffraction (Ghadiri et al. Nature 366:324-327 (1993)). By 1995, the Ghadiri group showed that the peptide rings can be covalently linked, thus trapping the dimeric form (Clark et al., JACS 117:12364-12365 (1995)). The authors expanded on this work demonstrating that dimers can be formed using photoswitchable linkers, where the linker is composed of an azobenzene subunit that leads to a reversible E to Z photoisomerization (Vollmer et al. Angew. Chem. Int. Ed. 38:1598-1601 (1999)). The authors note that the E isomer of the azobenzene exists as a diverse group of oligomers, formed as a result of the intermolecular hydrogen bonding between dimers, yet only a dimer pair is covalently linked. Again in 1999, the same group showed that the dimers can be created using iodine oxidation of cysteine residue in the monomers to form disulfide bonds or by using an olefin metathesis reaction involving the monomers to form a solely carbon-based linker between them (Clark et al. Chem. Eur. J. 5:782-792). These linked dimers are not inhibited by the kinetic instability of the non-covalently linked system and show some kinetic and thermodynamic stabilities. Non-linked cyclic peptide tubes are disclosed by Ghadiri for producing D,L peptide subunits (U.S. Pat. No. 6,613,875) and α-amino acids that have a repeating D,L unit or homochiral β-amino acids for use as antimicrobial peptides and compositions (U.S. patent application number US 2005/0107289).
Tubular assemblies of β-peptide rings are similar in fashion to those of α-peptides described by Ghadiri. Seebach et al. found that cyclic tetramers of 3-aminobutanoic acid exhibited X-ray powder diffraction patterns showing tubular crystal packing with nonlinear hydrogen bonding (Seebach et al. Helv. Chim. Acta 80:173-182 (1997)). The Kimura group synthesized a cyclic tri-β-peptide with trans-2-aminocyclohexylcarboxylic acid residues that self assembled into a rod-shaped molecular assembly (Fujimura et al. Org. Biomol. Chem. 4:1896-1901 (2006)). In 2007, Kimura demonstrated that the tube-like molecular assemblies can form from cyclic hexa-β-peptides (Hirata et al. Biopolymers 88:150-156 (2007)). The same group reported a cyclic tri-β-peptide system that contained terpyridine metal ligands that self assembled into tubes (Fujimura et al. Org. Lett. 9:793-796 (2007)). The terpyridine ligands bound the Cu(II) ions without disruption to the tubular assembly. McGimpsey disclosed cyclic D,L peptides and β peptides comprising of chromophore residues which possess electronic and electro-optic properties for the production of such devices (U.S. Pat. No. 6,902,720).
As best as can be determined, no prior research group has been able to link more than two peptide rings, due to the synthetic and structural complexity of the system. It appears that research on assemblies of peptide rings has been limited to noncovalent tube-like structures and linked dimer structures. It should be clear to one of skill in the art that a linked polymeric ring system would have enhanced properties when compared to an unlinked system.
With the aim to facilitate the use of the linked peptide scaffold as a tool in bionanotechnology, a method to produce peptide nanotubes polymers is presented. This disclosure describes a new method of covalently linking peptide rings in a way that prevents dimerization and allows for polymerization of the rings. This invention has implications not only in peptide and protein chemistries but also in the discovery and development of novel smart materials that are expected to show a number of enhanced properties when compared to natural biological systems.